Since fiber reinforced plastics using inorganic fibers as reinforcements are lightweight as well as are superior in tensile properties, compression properties and flexural properties, they have been utilized as structural materials in place of metallic materials in certain technical fields. Particularly, a fiber reinforced plastic using carbon fibers as its reinforcement has excellent lightweight properties in addition to the above properties and, therefore, it has been utilized as structural materials for aircraft as well as spacecraft such as artificial satellites and rockets, sports bicycles, wheelchairs, robots and the like.
However, a fiber reinforced plastic using carbon fiber as its reinforcement has a disadvantage that the plastic becomes brittle due to impact and, upon breakage, pieces thereof are scattered. The reason is that, in addition to extremely low elongation of carbon fibers, epoxy resin which is widely used as a matrix material of fiber reinforced plastics for structural materials is extremely brittle, while it is superior in heat resistance and chemical resistance.
In order to overcome such a disadvantage, from the viewpoint of reinforced fibers, improved carbon fibers having high strength and high elongation have been developed and, from the viewpoint of matrix resins, an improved epoxy resin having high elongation and maintaining its original physical properties has been developed. However, they have not yet fulfilled the requirements of the market.
Recently, in order to compensate for brittleness of carbon fibers by formation of a composite with other fibers, laminating of a reinforced resin layer using carbon fibers as the reinforcement and a reinforced resin layer using aramid fibers which have excellent impact resistance or ultra high molecular weight polyethylene fibers which have both high strength and high modulus as the reinforcement has been tried. Impact resistance of a fiber reinforced plastic composed of a reinforced resin layer using carbon fibers and a reinforced resin layer using aramid fibers is improved in comparison with that of a fiber reinforced plastic using carbon fibers alone as the reinforcement. On the other hand, in order to provide toughness required by the market thereto, it is necessary to considerably increase the amount of aramid fibers to be used. In this case, there are disadvantages that compression properties and flexural properties of the resulting fiber reinforced plastic are lowered and physical properties are deteriorated by water absorption under use conditions because water absorption of aramid fibers is as high as 3.5% under the standard conditions and, further, lightweight requirement can not be satisfied because the specific gravity of aramid fibers is 1.45, which is the highest among organic fibers.
On the other hand, a fiber reinforced plastic composed of a reinforced resin layer using carbon fibers and a reinforced resin layer using ultra high molecular weight polyethylene fibers has satisfactory lightweight properties, low water absorption and improved impact resistance because ultra high molecular weight polyethylene fibers are light and low in water absorption in comparison with aramid fibers and impact resistance is excellent. However, there is a disadvantage that delamination is liable to be caused by impact at the boundary between the reinforced resin layer of carbon fibers and that of ultra high molecular weight polyethylene fibers because of inferior adhesion to a matrix resin.